To be useful at high temperatures organic polymers must have a high softening point, e.g., a high glass transition temperature (Tg), and have sufficient thermal/oxidative stability (TOS) to retain adequate mechanical properties for a useful lifetime at expected service temperatures. Polyimide resins based on stoichiometrically balanced monomers of predominantly aromatic structure are known to fulfill such requirements. The high temperature performance of these polyimides has created considerable interest for their use in the aerospace industry. However, these polymers often require the use of extreme processing temperatures and pressures. Processing at extreme conditions is a deterrent to the manufacture of useful articles because of limited availability of such equipment.
Although the use of monomeric or low molecular weight prepolymer solutions facilitates impregnation of fiber structures, it still remains difficult to complete polymerization and devolatilization with this "wet" prepreg while achieving essentially complete consolidation, or compaction. This, in turn, has made it difficult to routinely produce high quality, low void, composite laminates with such resins, especially using the low to moderate pressure autoclaves which are typical of the aerospace industry composite manufacturing capability. The inherent processing difficulty of these resins arises from the high melt viscosity characteristic of the aromatic polyimide structure at chain molecular weights sufficient to yield high Tg.
In an effort to have low melt viscosity at the time of consolidation, and thereby ease or improve processability, several approaches have been developed in the past, but all of these have serious drawbacks. This invention overcomes many of these deficiencies.